Electrophotography



y 1969, YOSHIYUKI WATANABE ET AL 3,457,070

ELECTROPHOTOGRAPHY Filed July 13, 1965 2 Sheets-Sheet 1 FZ a mvrsmonsHam/Y m WATA/YABE By Kale/41 KIA/05m TA AT TORNEYS.

United States Patent US. Cl. 961.4 18 Claims ABSTRACT OF THE DISCLOSUREA method of electrophotography in which an electrostatic latent image isprovided on an element made up of a thin electrically insulatingtransparent layer that is bonded throughout its entire area to aphotosensitive layer, the element having a conductive electrode on theside of the photosensitive layer opposite the transparent insulatinglayer. The latent image is formed on tthe transparent insulating layerby first subjecting the element to an electrical field to depositcharges of one polarity on the element, thereafter illuminating theelement with a light image while subjecting the element to an electricalfield to the deposit charges of a polarity opposite to the firstpolarity on the insulating layer, stopping the illumination of theelement with the image and removing the electrical field. The latentimage is characterized by not being erased by subsequent exposeure toambient light.

The present invention relates to improvements in electrophotographyaccording to which a latent image impressed as an electrical chargepattern on a photosensitive plate is developed by applying finelydivided particles to the charged areas.

The present invention provides important advantages over known systemsfor electrophotography. The known systems of electrophotography may becategorized into the following four types in accordance with the basicprinciple underlying each such type.

The first involves the so-called xerography or electrofax processes inwhich a surface charge initially induced on the surface of a sensitiveelement is locally and selectively released under irradiation by light.Such systems are described in US. Patents Nos. 2,221,776 and 2,727,808.

The second involves a process such as that described in United StatesPatent 2,825,814 wherein a layer of photoconductive material is used asa photoelectric conversion element to obtain an induced electrostaticcharge pattern on a specially prepared recording paper.

The third involves a process, such as that disclosed in Japanese Patent3,017 of 1959, wherein an internal polarization is induced in anon-photosensitive recording plate of special construction to accomplishcopying, utilizing residual internal polarization effects.

The fourth involves the so-called persistent internal polarizationprocess, wherein a latent image on a phosphor material induced bypersistent internal polarization of such material exposed to light isutilized to effect copying.

Probably the most widely used of these four processes is Xerography, nowwidley commercialized. Electrophotographic processes employingxerography principles, however, require certain basic and exactinglimitations on the materials used as photosensitive elements, as theelectric current flowing through the photosensitive element is used toinduce required charge patterns. According to patent specificationswhich teach this process, it is a requirement of the process that aphotosensitive ele- 'ice ment be used having an ohmic resistance that ishigher in darkness, and that the electrostatic charges induced on thesurface of the photosensitive element be locally and selectivelyretained by making use of this characteristic of the photosensitiveelement. Additionally, the process, as an optical system, requires anelement of high photosensitivity, and the surface of the photosensitiveelement itself must exhibit photoconductivity. Thus, the processrequires integration of certain conditions which are essentiallycontradictory to each other to produce a practically operable system.The requirement that the ohmic resistance of the surface must be high ina dark environment, plus the requirement that the photosensitivity ofthe material must also be high, rigorously limits the selection ofmaterials which can be used for the element. As a practical matter,therefore, while a number of technical difficulties connected with themanufacture of suitable photosensitive elements were overcome, bytreating ZnO, or using spattered Se films to retain the mechanicalstrength of the element, low sensitivity remains as one of the seriousproblems. Another difficulty is the fact that the photosensitive elementmust be kept in the dark after the completion of the exposure operationuntil the developing operation is completed, since the latent image willbe lost if exposed to light.

The second system makes use of a phenomenon in which localized andselective irradiation by light induces transfer of electric charges fromthe photosensitive material to the recording plate when suchphotosensitive material is spaced from the recording plate by a narrowair gap of from about 2 to 10 microns in thickness. It is noteworthythat this narrow air gap operates as a specific type of resistancecomponent, and that the transfer of electrical charges does not occuruntil the electric field intensity selectively impressed across thisresistance attains a certain critical value; however, when such fieldintensity does reach this critical value, the migration of charges takesplace with abrupt intensity. The process makes use of this phenomenon inan effort to obtain a higher resolution of the copy. Conversely, thisphenomenon is obviously essential in this particular process to maintaina latent image induced on the recording plate. One of the majortechnical obstacles associated with this process is the necessity ofmaintaining such as critical narrow air gap between the photosensitiveelement and the recording plate.

The third system attempts to make use of the residual internalpolarization. In this process, the recording plate is made so thatprolonged retention of residual internal polarization is possible bymixing a specially selected material in the recording plate; thisenables retention of internal polarization of the recording plate thatis induced by a rise of electric field intensity in areas selectivelydistributed over the recording plate, which is in turn induced by theselectively localized decrease of field intensity impressed over thephotosensitive element at areas thereof exposed to light. Since thisprocess makes use of the internal polarization, it differs substantiallyfrom the above-described two processes, and particularly does notrequire the narrow air gap of the second process. This process,therefore, is less difficult to practice as a practical matter.

It must be noted, however, that only a limited selection of materials isavailable to provide the desired retention of the residual internalpolarization, which is the most important feature of this process.Furthermore, in actual practice, only a very short retention of internalpolarization can be achieved, and the process requires numeroustechnical refinements.

In contrast to the third process described above, which essentiallyattempts to make use of induced internal polarization in anon-photosensitive material, the fourth process, i.e., the so-calledpersistent internal polarization (PIP) process, makes use of aphenomenon known as persistent internal polarization induced within thephotosensitive element. By using a phosphor material and controlling itsdeep trap level, it is possible to induce a latent image on thephotosensitive plate and to retain such image in the dark. The PIPprocess not only requires that electrical charges be trapped at the traplevel in the areas exposed to light, but also requires that a certainamount of resistance be present in areas exposed to light. It appears,therefore, that photosensitive materials used for the PIP process areessentially limited to phosphor materials, thus indicating a majorobstacle for any attempt to improve the light sensitivity of theelements used in the PIP process. For example, CdSwhich is one of thephotosensitive materials preferably used in the present invention cannotbe used for the PIP process because it causes a high photosensitivity ofareas exposed to light, which in turn decreases localized distributionof electric field over the sensitive layer and increases the trapping ofelectric charges at the trap level. Consequently, the electric potential diiference between the areas exposed to light and the areas notexposed to light can be so little that an image-producing charge patternwill not result. The characteristics described above are generallycommon to all highly photosensitive materials, which indicates thathighspeed copying by the PIP process has major limitations.

One of the objects of the present invention is to pro vide an improvedelectrophotographic process and apparatus for eliminating basic defectsof electrophotographic copying processes with which the art is nowfamiliar, particularly those defects of the known processes relating tothe lack of photosensitivity, low strength of latent image and poorpersistence of latent image.

Another object of this invention is to provide electrophotographiccopying processes by which intense latent images can be formed byrelatively simple operations.

Still another object of this invention is to provide a novelphotosensitive element using photoconductive materials of highsensitivity that heretofore have been unusable in the field ofelectrophotographic copying, as well as certain methods whereby todetermine the operative characteristics of such photosensitive elements.

Still another object of the present invention is to provide certaincontrol methods whereby light images are converted intoelectrophotographic latent images by using such a novel photosensitiveelement.

In the present invention, problems like those discussed above areavoided or solved. Desired contact of a transparent electrode and theresistance surface of the photosensitive layer can be achieved by asimple application of pressure. It is immaterial according to theinvention whether there is an air gap that makes it diflicult toprecipitate a release of surface charges, or whether the transparentelectrode and the insulating layer of the element are so close thatcharges forming the latent image can easily migrate into the transparentelectrode. In any case, the image-inducing and image-retaining mechanismof the present invention can operate exactly in the same manner.

The present invention relates to the formation of a latent image,defined by electrical charge patterns corresponding to the light image,directly on a thin layer of material having high electrical insulationproperties and comprising an integral portion of a photosensitiveelement. The latent image may be developed by applying finely dividedparticles of any suitable dry developing material to the charge patternsand transferring the resulting visible image to another surface, such asthe surface of a recording paper sheet. Use of a special recording paperis not required. The latent image does not scatter or disappear evenwhen the element containing such latent image is subsequently exposed tolight.

The foregoing and other objects, advantages and fealllIG of th pr sentin ention will b app e t om the .4 following description of severalembodiments of the invention in connection with the accompanyingdrawings in which:

FIGURE 1 is an example of a photosensitive element used in the presentinvention, with a section thereof broken away to show its construction;

FIGURE 2 is a schematic side view of apparatus for forming a chargedlatent image on the photosensitive element, according to this invention;

FIGURE 3 is a graphic illustration of an example of relative values ofimpressed voltage and illumination by light;

FIGURE 4 graphically illustrates changes in relation to time ofelectrical charge potential on the surface of a photosensitive elementused in this invention;

FIGURE 5 diagrammatically illustrates means, and an operation, fordeveloping the latent image on the photosensitive element by applicationof dry developer particles;

FIGURE 6 diagrammatically illustrates means and operations fortransferring the latent image to a sheet of paper and fixing it on thepaper;

FIGURES 7, 8 and 9 are, respectively, a representation of an equivalentcircuit and characteristic curves illustrating the operation of thephotosensitive element embodying the invention;

FIGURE 10 is a schematic drawing illustrating an example of theelectrical potential distribution in a photosensitive element of theinvention;

FIGURE 11 is another example of relative values of impressed voltage andillumination of light;

FIGURES 12A and 12B are graphic illustrations of still another exampleof the relationship to time of electrical charge potential on thesurface of a photosensitive element in this invention;

FIGURE 13 is another example of a photosensitive element embodying thisinvention with a section thereof broken away to show its construction;

FIGURE 14 is a graphic illustration of another example of relativevalues of impressed voltage and illumination of light, for aphotosensitive element intended for a particular use.

Example 1 A photosensitive element 1 embodying the invention, shown inFIGURE 1, is of laminated construction and comprises a photosensitivelayer 2, a thin layer of a material of high electrical resistivity(hereinafter sometimes referred to as an insulating layer) 3 and anelectrode 4. The photosensitive layer 2 is a layer preferably of aboutmicron thickness formed by bonding crystals of copper (Cu)-activatedcadmium sulfide (CdS) and having grain size of approximately 10 microns,with cellulose acetate as the adhesive bonding agent. The thininsulating layer 3 is preferably a 12.5 micron thick layer of, forexample, solid, transparent polyester syntheic resin material; theinsulating layer 3 is laminated to one of the surfaces of thephotosensitive layer 2 by a polyester synthetic resin adhesivetransparent to light. An electrode 4 of a thin layer of flexible,electrically conductive material, such as aluminum, copper or silversheet or foil, is firmly laminated by an electrically conductiveadhesive, for example, an acrylic resin adhesive containing silverparticles, to the side of the photosensitive layer 2 opposite the sidethereof laminated to the insulating layer 3. Since all layers comprisingthe element are flexible, the integral element 1 of this embodimentitself retains substantial flexibility.

FIGURE 2 is a schematic view of an electrophotographic system embodyingthe present invention for inducing a latent image or charge patterncorresponding to a light image impressed on the element 1. An electrode5 of glass that is transparent to light and electrically conductive,such, for example, as NESA glass made by orning Glass Works of Corning,NY is di p h a conductive surface 5a in contact with the surface of theinsulating layer 3 of the element 1. A direct current (DC) supply source6 impresses an electric potential across the transparent electrode 5between its conductive surface 511 and the electrode 4 of element 1. Aswitching means 7 is connected between the current source 6 and element1 to connect or disconnect the electric current, and to change thepolarity of the electric potential so impressed across the transparentelectrode 5 and the electrode 4. Preferably, the element 1 isresiliently pressed against the transparent electrode 5 so thatsubstantially the entire surface of the insulating layer 3 is in contactwith the electrically conductive surface 5a of the transparent electrode5. An optical system 8 projects a light image of an object 9 to bereproduced against the surface of the element 1 through the transparentelectrode 5. Separate light sources 10 can be provided to illuminate theobject 9. It is understood that the device shown in FIGURE 2 representsbut one of many possible devices by which the process of the presentinvention can be carried out.

The manner in which a latent image is produced in the element 1 isgraphically illustrated in FIGURE 3 in which the ordinate in the upperpart of the figure is the electric potential impressed between thetransparent electrode 5 and the electrode layer 4 of the element 1, andthe ordinate in the lower part of the figure represents the lightprojected on element 1. The abscissae represent time in each case. Asshown in FIGURE 3, during passage of time from t to t an electricpotential is applied across the transparent electrode 5 and theelectrode layer 4 in such a manner that the polarity of electrode 5 ispositive with respect to the layer 4. The element 1 is not at this timeexposed to light. At time t the polarity of the electric potentialapplied is reversed so that the polarity at the transparent electrode 5is negative with respect to layer 4; this negative potential ismaintained from time t to time t during which time the element 1 isexposed to a light image as shown in the lower part of FIGURE 3. At timet the electrical potential is removed and exposure to the light image isstopped. Thereafter, the transparent electrode 5 is removed from theelement 1. The surface of the insulating layer 3 of the element 1 atthis time contains an electrical charge pattern forming a latent imagewhich corresponds to the incident light image to which the element 1 wasexposed during the period t to t After the removal of the transparentelectrode 5 from the element 1 as above described, the latent image soformed demonstrates almost no attenuation even under a subsequentexposure to light, nor is there any diffusion of charges comprising suchlatent image.

In an actual example, the time intervals t to t and t to t were both 0.1second, and the intensity of the light image projected against theelement 1 was 20 lux. Under such exposure conditions, when the voltagesupplied by the DC supply source '6 was 2,000 volts, the surface areasof the insulating layer 3 exposed to light were charged with electricalpotential of l,500 volts, whereas the electrical charge potential of theareas not exposed to light remained substantially at zero level.

The polarity of the surface charge of the element 1 thus induced byimpression of an electric field is the same as that of the transparentelectrode 5, which indicates that the latent image formed on the element1 is not formed by internal polarization, per se, of the element, butsubstantially by electrostatic charging of the surface of the insulatinglayer of the element 1.

FIGURE 4- graphically illustrates the changes of electrical potential inthe element 1 with respect to time in this embodiment of the invention.The symbols t t and t represent respectively the same time sequencesdescribed at t t and t in connection with FIGURE 3. Curve a depictschanges in electrical charge potential on the upper surface of theelement 1 during the interval t through 2 The broken line a connected tocurve a indicates that continued impression of the electrical potentialhaving the same polarity will not further increase the surface chargepotential of the element 1. Curve b represents the change on the surfaceelectrical potential of the element 1 at areas where the incident lightdoes not strike the element surface during the time interval representedby t through t Curve 0 represents the changes in the surface electricalpotential of the element 1 at areas where incident light strikes theelement surface during the time interval t through tg.

When impression of the electrical potential is continued beyond t curveb continues to drop downward at a change rate which is substantiallysmaller than the change rate of drop observed during the time interval tthrough t as shown by broken line b thus approaching, but gradually,curve 0; and curve 0, as indicated by broken line c, shows noappreciable drop even when impression of the electrical potential iscontinued beyond t Line G represents the electrical potential differenceat time t of the element 1 between the areas exposed to incident lightand the areas not exposed to incident light; this also represents thelatent image intensity defined by the electrical charge pattern on theelement 1 at time 1 The electrical charge pattern induced as a latentimage on the element 1 can readily be developed to a visible image byapplying a finely divided charged dry developer of a suitable known typethat is electrostatically attractable to the exposed or non-exposedareas to produce a visible deposited image. For example, as a negativelycharged toner, carbon black impregnated styrene resin may be used; andas a positively charged toner, a thermoplastic, methylated paraflinicresin, essentially composed by polymers of pinene, such as Piccopaleresin manufactured by Pennsylvania Industrial Chemical Corp. ofClairton, Pa., impregnated with oil black, may be used. In either case,powdered iron may be used as carriers for the toner. Particularlybecause of the high potential difference between the areas charged andthe areas substantially devoid of charges, the visible deposited imagewill have a fine resolution and will provide excellent reproduction ofthe latent image on the element. In the examples given, the negativelycharged toner will adhere to the areas that were not exposed to light,while the positively charged toner will adhere to the areas that wereexposed to light. This can be accomplished, for example, as shown inFIGURE 5, by placing the charged photosensitive element 1 with itscharged surface facing down over a container 21 containing the tonerparticles admixed in a suitable carrier 22 that are moved by agitator 23into contact with the element to form an image 24. If necessary ordesired, undesired particles can be removed by conventional means, as byblowing air. As shown in FIGURE 6, the visible image can then be readilytransferred to a sheet 25 of paper or the like by pressing the paperagainst the surface of the element 1 carrying the visible image totransfer the particles forming the image to the paper by light pressureapplied by an electrode 26, preferably a roller electrode, electricallyconnected to backing electrode 4 of element 1. The paper can then bepassed to a processor 27 that sets the image on the paper, as by fusingit in the conventional manner by known means.

There is offered here a theoretical explanation regarding the formationof the latent image and the reasons why processes embodying the presentinvention operate effectively while utilizing highly photosensitivematerials as the sensitive element. It is to be understood, however,that the theoretical explanation offered herein merely represents one ofthe possible elucidations of the phonemena utilized in the presentinvention and any different between the explanation herein offered andthose which may hereafter be offered will not in any way alter or modifythe substance of the present invention.

FIGURE 7 depicts an equivalent circuit, representing the light sensitiveelement 1 when an electric potential E is impressed on the element, inwhich Z represents the impedance component of the transparent insulatinglayer 3 comprising the surface of the element 1, and Z represents theimpedance component of the light sensitive layer 2. A DC field impressedon layers 2 and 3 from outside of the layers is essentiallyelectrostatic in nature. Such DC field will be distributed according tothe impedance present, because the light sensitive element of thepresent invention exhibits extremely high resistivity and current flowis'minimal. In the element 1 used in Example 1, the polyester resin filmconstituting the thin insulating layer 3 had a high unit arearesistivity of approximately l.25 ohms per square centimeter and adielectric constant of 3.1. The light sensitive layer 2 exhibited, afterhaving been stored in the dark for a prolonged period of time, a unitarea resistivity of about 1x 10 ohms, which reduced to approximately 1X10 ohms when exposed to light having intensity of 20 lux. The dielectricconstant of layer 2, determined by a method generally used, was 2.4 inthe dark, which increased to as high as 30 or more when exposed to lighthaving intensity of about 20 lux.

Accordingly, when an electric potential of about 2,000 volts isimpressed across the element, the potential which theoreticallyimpressed on the insulating layer 3 in the darkness should be about 14%of the impressed voltage, which should increase up to about 66% when theelement is exposed to 20 lux light. In practice, however, a large amountof field intensity is lost across the adhesive layer and the laminationsurfaces, which act as resistances. The charge potential induced overthe surface of the insulating layer 3 depends upon the intensity of theelectric field impressed across the element. When the impressed voltageis high, the charge potential induced on the insulating layer surface ishigh; and conversely, when the impressed voltage is lower, the potentialat the insulating layer surface is also lower.

Although the potential distribution at each layer comprising the lightsensitive element is as discussed above, the electric potentialappearing on the surface of the insulating layer 3 of the elementexhibits peculiar characteristic curves shown in FIGURE 8, whichillustrates the changes in the surface potential of the insulating layer3 of the element 1 with respect to time when an electric field isimpressed on the element. Curves D and D show the changes exhibited whenrespectively positive and negative potential is applied to thetransparent electrode 5 while the element 1 is in the dark. Curves L andL show the changes exhibited when respectively positive and negativepotential is applied to the transparent electrode 5 while element 1 isexposed to light. As apparent from these curves, each curve converges toa specific value when impression of the electric potential is continuedover a length of time. However, in practice, the respective values towhich said curves converge do not coincide with the changes in thedielectric constant and impedance referred to in the discussion ofFIGURE 7 and as between L curves and D curves, the surface potentiallevels shown by the curves illustrating those of the unexposed element(D curves) does not appear proportionate to the difference of thepotential between the exposed and the unexposed element as may betheoretically derived from the changes of impedance and dielectricconstant of the sensitive element, and appears somewhat higher than theexpected value.

component as described above, it would be more realistic I to considerthat the layer functions essentially as a ca pacitive component,Furthermore, although the light sensitive layer 2 in its unexposed stateexhibits a very high resistance', it would function as a capacitivecomponent also. Since one end of each such capacitive component isdirectly connected to an electrode, its charging time should beextremely short. In FIGURE 8 the length of the time required to have thesurface potential saturated to a specific value is shown as T, which interms of the photosensitive element described in Example 1 wasapproximately 0.1 second. However, when the charging time is computed,assuming that pure capacitive components alone were involved, theresults indicate that the saturation should be reached in much shortertime than the actual saturation time given above. The fact that it takesa longer period of time for saturation, coupled with the fact that thelevel of the electric potential of the unexposed element is notdetermined by the impedance and dielectric constant of the element 1,appears to suggest occurrence of unique potential changes within thelight sensitive layer 2.

We have investigated into possible reason why the preexposed elementwould exhibit such a potential level which does not conform to theimpedance and the dielectric constant of the element.

The following consideration is offered to explain this phenomenon. It isknown that the sensitivity of photoconductive materials is maintained byadding suitable activators. It is also known that the Cu-activated CdSemployed in the photosensitive layer 2 of Example 1 has a large numberof trap levels at the depth of about 1.4 electron-volts (ev.) and 0.03ev. from the bottom of the conduction band. The fact that CdS has twosuch trap levels, one shallow and another deep, suggests an importantbasis for considering the photoelectric current response to theincidence of light manifested in a photoconductive system employing CdS,and it bears close relevancy to the present invention as explainedbelow.

When a photoconductive material is exposed to illumination of light,while impressing a DC potential, a certain persistence characteristic isobserved, i.e., the photoelectric current will continue to flow for alength of time even after the excitation is removed. In case of CdSzCu,such persistence characteristic is particularly prolonged, as confirmedby experimental results. It is also clear that this characteristic isdue to Cu added as the activator to CdS crystal for the purpose ofincreasing, upon exposure to light, the density of charged carriers inthe conduction band within CdS crystal which participate inphotoconductive effect.

Such higher level of potential observed on the unexposed element isobviously undesirable in electrocopying, because the phenomenon onlycontributes toward the lowering of the potential difference between theareas of the element exposed to light and those not exposed to light,which means that the signal-to-noise ratio of the element leaves much tobe desired.

We have discovered that the foregoing disadvantage can be overcome byapplying an electric field of the inverse polarity to the elementimmediately prior to the exposure of the element to light signal, whichforms an important and essential element of the present invention, andWe have further discovered that by taking this step, any effect from theprior history of light irradiation on the element can effectively beeliminated, with a result that the level of the electric potential ofthe unexposed areas of the element can be held at substantially thevalue of the socalled dark current, while the potential level of theexposed areas of the element remains virtually the same with or withoutsuch pre-exposure impression of the inverse polarity, thus enabling oneto obtain an optimum signal-tonoise ratio in forming a latent image onthe photosensitive element. Such effect of the pre-exposure impressionof the inverse field will become readily apparent from the followingexplanations.

When excitation by light is removed while continuing to impress a DCpotential across a photosensitive layer 2 of CdS:Cu, electrons will bepresent in the conduction band for a considerable length of time, andthe impedance of the crystals rises but gradually. If in the course ofsuch gradual rise of impedance, when the polarity of the DC fieldimpressed on the layer is abruptly reversed, sudden increase ofimpedance will occur. FIGURE 9 illustrates this change in terms ofcurrent flow. The potential impressed in FIGURE 9 has the same polarityduring the time intervals respectively designated as t to t and thenbeyond t During the time interval shown at t to t the polarity isreversed. Excitation by light is performed during the interval shown ast to t A decrease of current fiow commences immediately after theremoval of excitation, and would follow the curve indicated by thebroken line except for reversal of the polarity t When the polarity ofthe field is reversed at t there is an instantaneous substantial flow ofcurrent, which decreases rapidly and settles down to a flow of smallamount. When the polarity is again reversed at i back to the originalpolarity, an instantaneous substantial flow of sudden current is againobserved, which attenuates rapidly in a short period of time and reachesthe value of the so-called dark current.

This phenomenon is observed more markedly when either both or one of thesurfaces of the sensitive layer 2 is coated with a thin layer of highlyresistive material. When only one of the surfaces of the sensitive layeris coated with the insulating layer, however, the changes observed willnot be symmetrical depending upon the polarity of the potentialimpressed. The type of current fluctuation described above are moremarkedly observed in a photoconductive layer 2 comprising particulatephotoconductive material crystals bonded together into a thin film byelectrically insulating adhesive, rather than a layer formed ofmonocrystals or spattered films. This phenomenon is important as itcontrols the impedance of the sensitive layer.

Utilizing the embodiment described in Example 1, We offer hereunder thetheoretical explanation of occurrences in the element 1 at each of thestages of the latent image formation steps already described inreference to FIGURE 3 and FIGURE 4. Some explanation of the imageretention mechanism of the element 1, particularly with respect to theimportant feature of the present invention, namely, that the latentimage formed on the surface of the insulating layer 3 will not bedestroyed after the removal of the transparent electrode 5 by anysubsequent illumination of the element 1 by light, will also be made.

During the time interval of t to t in FIGURE 4, the residual effect ofthe light to which the CdS crystals of layer 2 may have been previouslyexposed is predominant, and the density of electrons within theconduction band or in the trap levels, which can easily liberateelectrons into the conduction band, is quite high. By applying a DCcurrent having a definite polarity to the sensitive layer 2, theconduction electrons readily drift toward the positive pole and thusform internal polarization. The polarization of the sensitive layer 2thus effected, as shown in FIGURE 10, is in the direction opposite tothe polarity of the electric field externally applied, and the chargedistribution within the sensitive layer 2 will be as shown in thefigure. At this time because of their polarized distribution, electronsin the conduction band recombine at a low rate and do not decreaserapidly. Even though the field distribution as defined by the intrinsicimpedance and dielectric constant appears to require that most of thefield is impressed on the impedance component Z of the sensitive layer2, and only a small part thereof on the impedance component Z of theinsulating layer 3, because of the polarized charges induced within thesensitive layer, there will occur a corresponding increase in theintensity of the field distributed to the impedance component Z of theinsulating layer, which accounts for a higher charge potential whichappears on the surface of the insulating layer as shown in FIGURE 4.

This may also be explained by saying that there will be an apparentreduction of impedance component Z; or increase of dielectric constantof the photosensitive layer 2 because of its prior history of exposure.What is of particular importance is the fact that the internalpolarization of the sensitive layer is not the only agent whichmaintains the field equilibrium, but accumulation of charges havingopposite polarity on the reverse side of the insulating layer as well ason the boundary surfaces of photosensitive crystals and bonding agentalso contributes in the maintenance of this balance.

With respect to time interval t to 1 of FIGURE 4, the occurrences in theareas not exposed to light will be discussed first. In this timeinterval, conduction electrons still remaining in the conduction band orconduction electrons among the charged carriers distributed by internalpolarization liberated from the trap levels due to field emissioninduced by the field change, migrate within the conduction band towardthe newly set up field in the direction opposite to the direction ofdrift observed in the time interval t to t There will be a rise in thedensity of recombining, and the probability of recombination ofelectrons (which was low during the time interval t to t increasessuddenly, thus sharply reducing the number of electrons participating inconduction. No additional polarization results, so that immediatelyafter the impression of reverse voltage, there will be an abruptimpression of electric field in the insulating layer 3 of polarityopposite that of the field impressed during the tiume interval of t to tas shown by curve b in FIG- URE 4.

When a certain point is reached, there will be a decrease in the rate offield build-up and it continues but at a markedly slow rate, and as thepolarization set up during the interval of t to t becomes depolarized byheat or other causes, polarization in a new direction proceeds, but onlygradually, thereby slowly building up the surface charge. This meansthat the phenomenon is equally explicable as a rapid apparent increasein the impedance component Z of the sensitive layer.

The areas exposed to light during the time interval of f to t will nowbe discussed. The polarization which has been induced during the timeinterval of t to I is completely depolarized by the incident light. Theimpedance component Z of the sensitive layer 2 drops, and the dielectricconstant increases as a result, which also increases the intensity ofthe field impressed on the insulating layer. This in turn induces a risein the charge potential at the surface of the insulating layer. Thepolarized carriers in the sensitive layer will become trapped accordingto the polarity of newly impressed field, and at the same time, therewill be induced within the sensitive layer and the insulating layercertain internal charging, which occurs completely independently of thetrapping of charged carriers.

Both at the areas unexposed to incident light and at the areas exposedto incident light, the internal polarization and internal chargesinduced will operate to maintain the equilibrium with the charges on thesurface of the insulating layer 3. Therefore, even when the electricfield is removed as the next step of operation, this equilibrium doesnot go out of balance. This is one of the important features of thisinvention. In other words, if it was assumed that the retention of suchcharge balance of the photosensitive element is not possible, then whenthe field is removed the surface charges would cancel out across thetransparent electrode 5 with which the surface of the insulating layer 3is closely in contact, and the areas exposed and unexposed would settleinto the same potential level. Actually, when the transparent electrode5 and the insulating layer 3 are in contact, the absence of internalcharges of opposite polarity will result in the homogenizing of thesurface charges. Of course, the internal field induced by the trappedcarriers tends to disappear with passage of time, which causes changesin internal charge structure, bringing about corresponding changes inthe surface charges present on the photosensitive element.

As a next step, the transparent electrode is removed from the lightsensitive element. As discussed, there is an internal distribution ofcharges within the element which maintains an equilibrium with thesurface charges, so that even if transparent electrode 5 is removedunevenly from the element, any capacitive changes induced by such unevenpeeling will hardly cause redistribution of charge pattern, and thelatent image remains unperturbed. Since surface resistivity of theinsulating layer of the element is selected to be quite high, itprevents surface diffusion of such charges.

Accordingly, after the removal of the transparent electrode, it is notnecessary for the element 1 to have any internal mechanism to retain thelatent image induced on the surface of the insulating layer. Anysubsequent exposure to light of the element will not affect the chargedistribution on the surface 'of the element. The subsequent developmentoperations can be handled in light. Furthermore, the latent image onelement 1 will be retained semi-permanently, until such time as theelement is again exposed to a new electric field. What is particularlynoteworthy in the photosensitive element 1 is the fact that the chargesof opposite polarity internally present in the element do not extendtheir influence externally because of the backing electrode 4 integrallycomprising the sensitive element. The presence of the backing electrodeadditionally suggests a number of interesting technical possibilities.

As described above, the latent image defined by the charge patternpresent on the surface of the insulating layer 3 interacts with theinternal charges and shows hardly any attenuation. After a simpleexposure to light to depolarize the internal polarization which may havebeen present from the prior use of the element, when an electric fieldis impressed across the element, the entire process as described abovecommencing with t to and thereafter can be repeated. Accordingly, evenwhen charges are present on the surface of element 1, such charges willnot affect in any way the subsequent formation of a fresh latent imageon the element. The element will be subjected to certain mechanicalstimuli, but such stimuli do not extend their effect to the sensitivelayer, and the effect will be limited to the thin insulating layer,which indicates that the possibility of fatigue or mechanical breakdownof the photosensitive layer is nil.

From the foregoing it is apparent that the charge distribution withinthe photosensitive layer of the element causes changes in the effectivefield distributed on the surface of the insulating layer and, withoutthe provision of any air gap between the element and the transparentelectrode or without special conditioning of the insulating layer,induces an electrostatic charge pattern forming a latent image of thepattern of incident light on the surface of the insulating layer.

Particularly noteworthy is the fact that the contacting of thetransparent electrode and the photosensitive element does not requirespecial skill or arrangement. It has been found that after theelectrostatic charges have been induced according to the proceduredescribed above, when the transparent electrode 5 and the electrode 4laminated to the photosensitive element are short-circuited beforeremoving the transparent electrode from the element, the latent image isdestroyed. Such destruction of the image occurs as a result of a suddendrop of the surface potential of the element to zero level, andindicates that the electric charges present at the surface of theelement are dispersed through the short-circuiting. In this case, theinternal charge potential of the element which interacted to retain theequilibrium with the surface potential will also be destroyed.Considering this in terms of the polarity of the latent image and thepolarity of the electric field impressed, it appears that the latentimage is formed electrostatically, and that space at t e contactingsurfaces of the transparent electrode and the insulating layer will notprevent transfer of charged carriers between the contacting electrodeand the surface of the element. As a practical matter, any space thatmay be present between the transparent electrode 5 and the surface ofthe insulating layer 4 of the element 1 does not affect the imageformation, and it has been ex perimentally established that it issufficient that the transparent electrode be in contact with the elementwith appreciable mechanical pressure. Conversely, image formation asdescribed above will not be substantially adversely affected even ifspace may exist between the transparent electrode and the element.

The electric charges within the sensitive layer act as the retainer andcontroller of the charges distributed over the insulating layer surface,as indicated below.

After electrostatic charges corresponding to the light image have beeninduced on the insulating layer 3 of the element 1, when light isirradiated before the transparent electrode 5 is removed from theelement but without short-circuiting the electrode 4 of the element andthe transparent electrode 5, the potential difference between theexposed areas and unexposed areas will disappear, although the averagepotential over the entire surface drops only slightly, so it is notpossible to develop the image. This indicates that the latent imageformation mechanism will become lost as a result of the equalization ofcharge distribution induced by exposure to light. Thereafter, when thetransparent electrode and the electrode of the element areshort-circuited, the surface potential, like in the previous case, dropscompletely to zero level.

As indicated above, the construction of the photosensitive element andthe control method thereof in the present invention make possible theformation of a stable latent image while utilizing high sensitivityphotoconductive materials which have not heretofore been used becausetheir characteristics, manifested because they are highly sensitive tolight, made them unfit for prior electrophotographic processes.

As discussed in Example 1, factors which contribute in latent imageformation are the internal polarization and the charge distributions onthe reverse side of the insulating layer or in the sensitive layerhaving a polarity which is the reverse of that of the charge patternpresent on the surface of the insulating layer which defines the latentimage. The internal polarization, as already indicated, contributes ininducing field distribution at the time the latent image is formed andin the image retention thereafter until and only until the transparentelectrode is removed from the surface of the photosensitive element. Theinternal polarization may disappear at any time after the removal of thetransparent electrode, without in any way. affecting the latent imageformed on the surface of the insulating layer. In other words, thepolarization need not be as deep in the trap level as would benecessitated when persistent internal polarization is desired, and thisforms an important feature of the present invention as it eliminates thedifficulties of selecting photosensitive materials suitable for thepersistent internal polarization processes. Put in another way,generally, a material which exhibits persistent internal polarizationeffects would have a low photosensitivity and low impedance changecharacteristics except those resulting from the internal polarizationeffect. On the other hand, in the present invention materials of highphotosensitivity and high impedance change characteristics may be used,thereby obtaining a very fast electrophotographic plate. However, theuse of materials which exhibits persistent internal polarization effectsare not precluded from the present invention, as shown in the followingexample.

Example 2 ZnCds in powder form, silver (Ag) activated and having a grainsize of about 5 microns, was molded into a thin layer 2 of about 50micron thickness using cellulose acetate as the bonding agent. To thereverse side of the photosensitive layer thus formed an electrode 4 madefrom thin aluminum film, and on the obverse side an insulating layermade from 12.5 micron thick polyester resin film, were respectivelylaminated by suitable adhesive. This constituted the photosensitiveelement 1. The transparent electrode 5 and the DC current source 6 weresubstantially the same as those of Example 1.

During the image formation process, as shown in FIGURE 11, the element 1was illuminated substantially uniformly over the entire surface by lightduring the time interval t to t while at the same time a voltage wasimpressed across the two electrodes such that the polarity of thepotential was positive at the transparent electrode 5. This step wastaken to create an internal polarization of the entire surface of theelement 1 in a single uniform direction, thereby preparing the plate forsubsequent illumination by image. Thereafter, the polarity was reversedand during the time interval 1 to t the element was exposed to a lightimage. The prior illumination of the element was the only step which wasmaterially different from those employed in Example 1. The preexposureillumination lasted for 2 seconds, and the light image intensity at thetime of the exposure of the element was 20 luxes. When the externallyimpressed voltage was 2,000 volts, there was a surface charge of +200volts observed on the areas not exposed to light, and a charge of 400volts at the areas exposed to light, if subsequently treated by anotherillumination by light, as hereunder discussed.

FIGURE 12-A illustrates the changes in the surface potential at eachstep of the operation described, and FIGURE 12-B illustrates the changesin the surface potential when the element was not exposed to light atthe time the initial voltage was impressed across the element. As it isclear from a comparison of FIGURES 12-A and 12-B, the potentialdifference G between the exposed and unexposed areas, which determinesthe intensity of the latent image, is much greater in FIG- URE 12-A,which represents the operation in which the element was initiallyexposed to light when the field voltage was impressed. A uniquecharacteristic of this embodiment is the fact that the surface chargepotential registered appears quite low even after the element isseparated from the transparent electrode, if the element is notthereafter exposed to light. In the example given, whereas the surfacepotential at the exposed areas of the element measured prior to suchpost-exposure illumination by light was found to be -200 volts, the samewas measured at -400 volts after the element was so illuminated bylight.

This phenomenon apparently arises because the internal polarizationoperates as an effective image formation and image retention mechanismin the element used in Example 2, and such internal polarization isprimarily formed by electrons at the deeper trap levels that causepersistent internal polarization which will become depolarized by thepost-exposure illumination by light. In other words, even after thetransparent electrode is removed after the formation of latent image,the persistent internal polarization effects continue to persist andthus balances with the surface charge potential until the element isagain illuminated by light. When the element is so illuminated by lightafter the removal of the transparent electrode, and persistent internalpolarization killed, the surface charge effect alone would be extracted,and thus the apparent net surface charge would become intensified.However, when a phosphor material is used for photosensitive element asin Example 2, since the impedance fluctuation of such material is quitelow, it appears that the formation of internal polarization is theprimary factor contributing to the formation of a latent image and that,accordingly, a system having high light sensitivity will not result.However, such an element will be useful in recording images of radiantenergy other than the visible light.

An important difference between the processes of Example 1 and Example 2involves charges at the boundary surface regions of the light sensitivecrystals and bonding agent which are related to the changes in theintrinsic impedance of the light sensitive crystals.

In case of Example 1, during the interval of t to t (FIGURES 3 and 4),the impedance of crystals present in the photosensitive layer unexposedto light is quite high, so that it is probable that, at the boundarysurfaces of the light sensitive crystals, electric charges will build upwhich will correspond to the polarity of the externally applied electricfield. On the other hand, during the interval of t to in the areasexposed to light, the resistivity of the light sensitive crystalsgreatly decreases, which indicates that such electrostatic charges wouldhardly build up at the surface regions of the crystals exposed to light.Experimentally, when the surface charge of the areas unexposed to lightduring the time interval of t to t is measured after the removal of thetransparent electrode 5, but without subsequently exposing the elementto light, the measurement will give the value of approximately 10()volts; on the other hand, when the element is exposed to light after theremoval of the transparent electrode and then the surface charges of theunexposed area measured, the value obtained is in the neighborhood of 0volt, suggesting that the post-exposure illumination of the element(after the removal of the transparent electrode 5) will bring thesurface charges of the unexposed areas of the element to zero level. Thevalues of surface charges observed during the time interval t to t i.e.,during the time interval wherein a field having the polarity opposite tothat applied during the exposure, do not appreciably differ as betweenpreviously exposed areas and unexposed areas. Additionally, as shown bycurves L and L in FIGURE 8, the value of the surface charge of theexposed area, as measured after the exposure of the element followed byremoval of the transparent electrode 5, is exactly the same whether ornot the element is exposed to light after the removal of the transparentelectrode. This suggests that in case of Example 1 wherein highlysensitive photoconductive crystals in particulate form are used, nocharge build-up at the boundary surfaces of the photosensitive crystalswhere they are in contact with the bonding agent will occur in the areasexposed to light, and such charge build-up on the surface of theboundary regions will occur only in the areas unexposed to light duringthe time interval t to t In Example 2, however, the situation isdifferent, and it is apparent that the rate of electron trap will becomehigher in the boundary surface regions because conduction electrondensity becomes higher there even at the areas exposed to light, and theboundary surface regions maintain a high resistivity and do not permitfree migration of charged carriers at the surface of the crystals. As aresult, in Example 2, in which phosphor crystals in powder form are usedas the photosensitive material, the charge build-up may occur at theboundary surfaces of the photosensitive crystals where they are incontact with the bonding agent, irrespective of whether the crystalshave been exposed to light. In this case, the surface charges of theelement are not counterbalanced merely by reverse charges and theinternal polarization within the light sensitive crystals, but it shouldbe clear from the foregoing discussion that it also involves the chargesbuilt up at the boundary surface regions of the light sensitive crystalsand the bonding medium. Such charges built up in the boundary surfaceregions have a reverse polarity with respect to the polarity of theinternal polarization and operate to negate the internal polarizationfield induced within the crystals, and thus the total effect of suchcharges present at the boundary surface regions is detrimental toobtaining the optimum signal-to-noise ratio.

The foregoing comparison of the elements used in Example 1 (using ahighly photoconductivesubstance) and Example 2 (using a phosphor) willmake clear that the present invention does not rely solely upon thepersistent internal polarization principles. Because the internalpolarization principles fail to explain the relatively large fluctuationof the surface charge observed on the unexposed areas of the elementwhen the element, after the removal of the transparent electrode, isilluminated by light, despite the fact that the areas not exposed tolight during the time interval of t to t would have the least amount ofphotoconductive electrons and would have, accordingly, the leastopportunity to form internal polarization, we believe that, as discussedabove, the unexposed areas of the element described in Example 1 possesson the surface of the photoconductive crystals electric charges havingsuch polarity as will be dictated by the externally applied electricfield, which will disappear through recombination of electrons as aresult of the lowering of the resistivity of the crystals induced bylight to which the element is exposed after the removal of thetransparent electrode.

In inducing a latent image on the element utilized in Example 1, whenthe polarity of the voltage impressed is entirely reversed, i.e., when avoltage is impressed with the polarity of the transparent electrodenegative during the time interval t to t, and then positive during thetime interval t to t in FIGURES 3 and 4, both the photosensitivity ofthe system and the clarity of the latent image decrease. There will alsobe marked deterioration of the latent image when exposed to excesslight. This appears to be due to the unsymmetrical construction of thephotosensitive element, and appears to be attributable to a type ofrectification effect exhibited by the element; this characteristic isnot observed when a phosphor is used as in Example 2.

The elements 11 in Examples 3 and 4 below are more symmetrical inconstruction than the element 1 used in Example 1, and they remove thedisadvantages resulting from the asymmetrical construction utilized inExample 1. Examples 3 and 4 also prove (as a result of insertion ofanother insulating layer, sandwiching the light sensitive layer) thatthe working principles of the element of the present invention do notdepend on the transfer of charges carriers between the light sensitivelayer and the electrical field externally applied for effective latentimage formation, but that such formation of latent image on the surfaceof the insulating layer depends solely on the occurrences of certainphenomena, as described above, within the light sensitive layer, whichdistinguishes the invention from other dry electrophotographic methodswith which the art is now familiar.

Example 3 The only difference in the construction of the light sensi-Example 1 is, as shown in FIGURE 13, that another insulating layer 10,having the material characteristics and dimensions equal to those of theinsulating layer 3, was interposed between the sensitive layer 2 and theelectrode 4, so the light sensitive layer 2 had an insulating layer oneach side. Insulating layer was also integrally laminated to theelement. Latent image formation was successfully carried out on thiselement 11 by the steps exactly as described in Example 1. Thephotosensitivity of the element 11 was the same as the element describedin Example 1, and an exposure for 0.1 second to a light image of luxesobtained a latent image of usable strength. The exposure to light image,and the time sequency of field impression were the same as described inExample 1. When the voltage externally applied was 2,000 volts, as inExample 1, the charge potential of the element at the area exposed tolight was 1,200 volts, whereas the charge potential at the unexposedareas was subsentially zero. While there was thus a slight drop inefficiency, this was compensated by an advantage not demonstrated by theelement used in Example 1. The signal-to-noise ratio of the latent imagewas considerably improved compared with Example 1, which indicateselimination of the .socalled noise factor due to the highlyunsymmetrical structure of the element or of the low energy inputappearing as latent image. Particularly, the fact that noises due tounsymmetrical nature of the element are essentially eliminated has animportant practical significance.

Example 4 In this example, the same photosentive element 11 described inExample 3 and shown in FIGURE 13 was used. As shown in FIGURE 14, thesteps up to t starting from t were the same as those employed inExample 1. During the time interval to t without illuminating theelement with a light image, the electric field was removed, thetransparent electrode 5 temporarily separated from the element 11 andthe transparent electrode 5 was again contacted with element 11.Beginning at time t and continuing until time t the element wasilluminated with a light image, while an electric potential wasimpressed having a polarity the reverse of that of the potentialimpressed during the time interval t to t During the next time interval,t to t the light image was absent and only the electric potential havingthe same polarity as the field impressed during the interval t to t wasimpressed; at point t the element was separated from the transparentelectrode. The resulting latent image on the element 11 had almost thesame intensity and clarity as that obtained by Example 1.

The latent image produced in each of Examples 2 to 4, inclusive, can bedeveloped into a visible image and transferred to a sheet of paper, orthe like, by the means and methods discussed above in connection withExample 1.

As apparent from the above examples the characteristics of thephotosensitive materials used are an important feature of the invention.Generally speaking, when photosensitive material in powder form isbonded together in a thin layer by electrically insulating adhesive, andeither one or both surfaces of the layer is covered with thin layer ofelectrically insulating material, a sudden change of resistance willoccur when an electric field impressed across such element is reversed.This effect is more pronounced when an activating material is present inthe photosensitive material to activate its light sensitivity. Work onthis invention has shown that various kinds'of photoconductive materialsand phosphors may be used as the photosensitive material according tothe present invention; examples are CdS, ZnS, ZnO, CdSe, PbS, ZnSe,ZnTe, CdTe, and the like, preferably actuated by known materials foractuating photosensitivity, such as copper. Preferably these materialsare sufficiently finely divided to permit them to be formed into aphotosensitive layer of 200 microns or less in thickness, in which theparticles are held by a bonding agent. Such construction is preferablebecause it promotes sudden decrease of photoconductive electron densityon reversal of polarity and induction of internal polarization.

In such a construction, the selection of a bonding agent should be basedon the following considerations. The relative impedances and dielectricconstants of the sensitive layer and the insulating layer must beconsidered. As it is essential to confine the phenomenon occurring inthe photosensitive material in each particulate unit of such material inorder to obtain good image retention and higher resolution, it ispreferable to select the bonding agent so that the particles ofphotosensitive material are essentially uniformly separated anddispersed throughout the layer 2.

,F or these purposes, the selected bonding agent should have a specificvolume resistivity of at least 10 ohm-centimeters and be transparent tothe incident light used.

The thickness of the sensitive layer should also be considered inselecting the bonding agent for desired strength and mechanicalflexibility of the layer, as well as for the absorption of incidentlight.

On the basis of the resistivity of the sensitive film exhibited in thedark as well as in exposure to light, a suitable insulating layermaterial, which should be transparent to the incident light and whichshould allow most of the field to be impressed upon the sensitive layerwhen the sensitive layer is unexposed and most of the field to beimpressed upon the insulating layer when the sensitive layer is exposedto light, is selected and bonded to the sensitive layer by a suitableadhesive transparent to the incident light and having high electricalresistance.

Such insulating layer may be bonded to one side of the sensitive layer,in which case the opposite side of the sensitive layer should be bondedby a suitable adhesive to an electrically conductive electrode, or suchinsulating layer may be bonded to both sides of the sensitive layer, inwhich case the backing electrode should be bonded to one of the outsidesurfaces of such insulating layers sandwiching the sensitive layer.

The insulating layer should preferably be selected from a group ofmaterials having a volume resistivity of ohm-cm. or more and also havingsurface resistivity of 10 ohm-cm. or more. The surface-to-surfaceresistance per unit area of such insulating layer should be 10 ohms persquare centimeter or more, and dielectric constant thereof should besuch that when considered in terms of the dielectric constant of thesensitive layer, in a perfect darkness and having no prior history ofexposure to light, the intensity of electric field impressed to theinsulating layer in that condition should be at least equal to or lessthan that of the field which would be distributed to the sensitivelayer. Another consideration relating to the se lection of theinsulating layer material is that when the dielectric constant of thephotosensitive layer is increased as a result either of sudden increaseof conduction electrons within the sensitive layer or at the trap levelsinduced by exposure to light, the electric field impressed upon theinsulating layer should be markedly greater than that impressed upon thesensitive layer.

The light sensitive material made into a thin layer, as a whole, and theinsulating layer should satisfy the following requirements: First, asthe rate of light absorption determines the sensitivity of the layer,light absorption should be high, but a layer excessively thick withrespect to the incident light requires that a higher external voltage beimpressed on the element, and such excess thickness also operatesdetrimentally to the formation of charge on the surface of the element;accordingly, the sensitive layer should not be thicker than necessary.Secondly, the insulating layer should be selected so that the impedanceand dielectric constant thereof maintain discrete relationship to theimpedance and dielectric constant of the sensitive layer. This isessential in obtaining an element of high efficiency. Thirdly, because acharge distribution which is induced on the reverse side of theinsulating layer works advantageously for image retention, a betterresult is obtained if the material from which the bonding agent forsensitive layer is made is different from the material from which theinsulating layer is made. The selection of different materials for thebonding agent and the insulating layer is preferred also for the purposeof setting the relative impedance of the two materials in the mannerpreviously explained, and this offers an easier method of constructing asatisfactory photosensitive element.

Particular attention must be paid in bonding the light sensitive layerto the insulating layer. If the bonding achieved happens to be uneven orif the thickness of the adhesive is not uniform, it will constitute acause for local irregularity of the field distribution over the elementregardless of whether or not a satisfactory contact is achieved betweenthe transparent electrode and the element. When the sensitive layer 2and the insulating layer 3 are not integrally laminated but are onlyclosely in contact with one another, while in principle the element 1will perform the necessary function, it has been found that integrallamination of the sensitive layer and the insulating layer is preferred,particularly when the area of the element is large. Integralconstruction of the element is also preferred to prevent any mechanicalstimulus from intervening with the latent image formation of theelement.

In the foregoing specification, whenever the word light is used, itshould be understood to mean any radiant energy in the form ofelectromagnetic waves having the wavelength substantially that of gammarays up to and including infra-red rays of not more than 4 micronwavelength.

Modifications of the invention other than those indi cated above will beapparent to those skilled in the art.

It is intended that the patent shall cover, by suitable expression inthe appended claims, whatever features of patentable novelty reside inthe invention. The terms and expressions which have been employed areused as terms of description and not of limitation, and there is nointention of excluding such equivalents of the invention described or ofportions thereof as fall within the purview of the claims.

We claim:

1. A method of forming an electrostatic latent image comprising varyingelectric charges on an insulating surface, which method comprisesproviding an element made up of a thin electrically insulatingtransparent layer that is bonded to a thin layer comprising finelydivided particles of a material that develops electrical conductivitywhen exposed to light, which particles are bonded into such thin layerby a solid bonding agent that is electrically non-conductive buttransparent to light, which thin layer of particles is in inductiveproximity to another electrode on the side thereof opposite the side towhich said insulating layer is bonded, positioning a removablelight-transparent electrically conductive electrode in contact with saidinsulating transparent layer, impressing a direct current voltage acrosssaid transparent electrode and said other electrode, than reversing thepolarity of said direct current voltage While substantiallysimultaneously illuminating through said transparent electrode thesurface of said transparent insulating layer with a light image,extinguishing said illumination by said light image, ceasing impressionof said direct current voltage and removing said transparent electrodefrom said surface of said transparent insulating layer while notpermitting light to illuminate said layer of particles, thereby creatingon the surface of said transparent insulating layer a charge patternsubstantially defining a latent image representing the light image thatwas illuminated.

2. A method of electrophotography which comprises providing an elementcomposed of a photosensitive material in particulate form bonded into afilm not more than 200 microns thick by means of a high polymer bondingagent having a specific volume resisitivity of at least 10ohm-centimeters and transparent to light thereby providing aphotosensitive layer, a thin insulating layer not more than 50 micronsthick composed of a material selected from a group of materials beingtransparent to light and possessing a highly insulating property andhaving a specific volume resistivity of at least 10 ohm-centimeters anda specific surface resistivity of at least 10 ohms per squarecentimeter, said thin insulating layer having a surface-to-surfaceresistivity per unit area of at least 10 ohms per square centimeter andbeing bonded to said photosensitive layer and a electrode composed of anelectrically conductive material bonded to the surface of saidphotosensitive layer opposite said insulating film, disposing aremovable transparent electrode in surface-to-surface contact with saidthin insulating film, impressing a direct current voltage across saidtransparent electrode and said electrode bonded to said photosensitiveelement While not permitting light to illuminate said photosensitiveelement, then reversing the polarity of said direct current voltage: soimpressed while substantially at the same time illuminating the surfaceof said photosensitive element by a light image, then extinguishing saidillumination of said light image and removing the impression of saidvoltage at a time not substantially prior to extinguishing saidillumination, and removing said transparent electrode from said surfaceof said photosensitive element while not permitting light to illuminatesaid photosensitive element, thereby inducing on the surface of saidthin insulating layer a charge pattern substantially defining a latentimage being characterized by not being erased by subsequent exposure toambient light.

3. A method of electrophotography which comprises providing an elementcomposed of a photosensitive material in particulate form bonded into afilm not more than 200 microns thick by means of a high polymer bondingagent having a specific volume resistivity of at least ohm-centimetersand transparent to light thereby providing a photosensitive layer, athin insulating layer having not more than 50 microns in thickness of amaterial selected from a group of materials being transparent to lightand possessing highly insulating property and having a specific volumeresistivity of at least 10 ohms per square centimeter, said thininsulating layer having a surface-to-surface resistivity per unit areaof at least 10 ohms per square centimeter and being bonded to saidphotosensitive layer, said materials of said photosensitive layer andsaid thin insulating layer being selected such that the consumption ofelectricity by an electric field externally impressed upon said twolayers in said photosensitive layer at the time said photosensitivelayer is illuminated by light falling thereon is not greater than theconsumption of electricity by said field so impressed externally in saidthin insulating layer, and an electrode composed of an electricallyconductive material bonded to the surface of said photosensitive layeropposite said insulating layer, disposing a removable transparentelectrode in surface-to-surface contact with said thin insulating layer,impressing a direct current voltage across said transparent electrodeand said electrode integrally laminated to said photosensitive elementwhile not permitting light to illuminate said photosensitive element,then reversing the polarity of said direct current voltage so impreseedwhile substantially at the same time illuminating the surface of saidphotosensitive element by a light image through said transparentelectrode, then extinguishing said illumination of said light image andremoving the impression of said voltage at a time not prior toextinguishing said illumination, then removing said transparentelectrode from said surface of said photosensitive element while notpermitting light to illuminate said photosensitive element, therebyinducing on the surface of said thin insulating layer a charge patternsubstantially defining a latent image representing said light image soilluminated, said latent image being characterized by not being erasedby ambient light.

4. A method of electrophotography which comprises providing an elementcomposed of a photoconductive or phosphor material in particulate formbonded into a film not more than 200 microns thick by means of a highpolymer bonding agent having a specific volume resistivity of at least10 ohm-centimeters and transparent to light thereby providing aphotosensitive layer, a first thin insulating layer not more than 50microns thick composed of a material selected from a group of materialsbeing transparent to light and possessing highly insulatmg property andhaving a specific volume resistivity of at least 10 ohm-centimeters anda specific surface resistivity of at least 10 ohms per square centimeterprepared such that the surface-to-surface resistivity of said layer isat least 10 ohms per square centimeter and being bonded to saidphotosensitive layer, a second thin insulating layer composed of amaterial selected from a group of materials possessing highly insulatingproperty and having a specific volume resistivity of at least 10ohm-centimeters and a specific surface resistivity of at least 10 ohmsper square centimeter prepared such that the surface-to-surfaceresistivity per unit area of said layer is at least 10 ohms per squarecentimeter bonded to the other surface of said photosensitive layer, andan electrode composed of an electrically conductive material bonded tothe surface of the second insulating layer thereby providing aphotosensitive element, disposing a removable transparent electrode insurface-to-surface contact with said first thin insulating layer,impressing a direct current voltage across said transparent electrodeand said electrode integrally laminated to said photosensitive elementwhile not permitting light to illuminate said photosensitive element,then reversing the polarity of said direct current voltage so impressedwhile substantially at the same time illuminating the surface of saidphotosensitive element by a light image through said transparentelectrode, then extinguishing said illumination by said light image andremoving the impression of said voltage at a time not substantiallyprior to extinguishing said illumination, then removing said transparentelectrode from said surface of the first insulating layer while notpermitting light to illuminate said photosensitive element, therebyinducing on the surface of said first thin insulating layer of saidphotosensitive element a charge pattern substantially defining a latentimage representing said light image, said latent image beingcharacterized by not being erased by subsequent exposure to ambientlight.

5. The method of forming an electrostatic latent image embodying varyingelectric charges on a surface composed of insulating material whichcomprises providing an element made up of a thin electrically insulatinglayer directly bonded through its entire area to a photosensitive layer,subjecting said element to an electric field between an electrode on thesame side of said element as said insulating layer and an electrode onthe side of said element opposite said insulating layer to depositcharges of a first polarity on the element, thereafter illuminating theelement with a light image while subjecting the element to an electricfield between an electrode on the same side of said element as saidinsulating layer and an electrode on the side of said element oppositesaid insulating layer to deposit on the element charges of a polarityopposite to the first polarity, stopping the illumination of the elementwith the image and removing the electric field without shortcircuitingthe said electrodes between which the lastmentioned field was created,said latent image being characterized by not being erased by subsequentexposure to ambient light.

6. A method according to claim 5 in which the photosensitive element isnot illuminated during the time that the charges of the first polarityare deposited on the element.

7. A method according to claim 5 wherein the photosensitive element issubjected to substantially uniform illumination during the time that thecharges of the first polarity are deposited on said element.

'8. A method according to claim 5 in which the insulating layer istransparent.

9. A method according to claim 8 in which the element has a conductiveelectrode bonded on the side of the photosensitive layer opposite thetransparent insulating layer.

10. A method according to claim 9 in which another insulating layer isinterposed between the photosensitive layer and the conductiveelectrode.

11. The method of forming an electrostatic latent image embodyingvarying electric charges on a surface composed of insulating materialwhich comprises providing an element made up of a thin electricallyinsulating layer directly bonded through its entire area to aphotosensitive layer, subjecting said element to an electric fieldbetween an electrode on the same side of said element as said insulatinglayer and an electrode on the side of said element opposite saidinsulating layer to deposit charges of a first polarity on the element,thereafter illuminating the element with a light image while subjectingthe element to an electric field between an electrode on the same sideof said element as said insulating layer and an electrode on the side ofsaid element opposite said insulating layer to deposit on the elementcharges of a polarity opposite to the first polarity, stopping theillumination of the element with the image, removing the electric fieldwithout short-circuiting the said electrodes between which thelast-mentioned field was created and thereafter subjecting said elementto substantially uniform illumination thereby enhancing said latentimage, said latent image being characterized by not being erased bysubsequent exposure to ambient light.

12. A method according to claim 11 wherein the photosensitive element issubjected to substantially uniform illumination during the time that thecharges of the first polarity are deposited on said element.

13. A method according to claim 11 including the additional step ofdeveloping the latent image by the application of a toner thereto.

14. A method according to claim 13 in which the developed image issubsequently transferred to a sheet of paper or the like.

15. A method according to claim '11 in which the photosensitive elementis not illuminated during the time that the charges of the firstpolarity are deposited on the element.

References Cited UNITED STATES PATENTS 2,741,959 4/1956 Rheinfrank etal. 96-1.4 2,853,383 9/1958 Keck 96-1 2,901,348 8/1959 Dessauer et al96-1.5 2,912,592 11/ 1959 Mayer.

3,196,011 7/1965 Gunther et al. 96-1.1 3,268,331 8/1966 Harper 9613,308,233 3/1967 Button et a1 961 3,347,669 10/1967 Kalman 96-1 NORMANG. TORCHIN, Primary Examiner C. E. VAN HORN, Assistant Examiner U.S. C1.X.R.

27 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,457, 070 Dated July 22, 1969 Inventor(s) Yoshiyuki Watanabe and KoichiKinoshita It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

r- In the heading the name of the Assignee reading Matsuragawa ElectricCompany, Ltd.

should read Katsuragawa Electric Company, Ltd.

SIGNED AND SEALED OCT 2 1 I969 Attest:

mum E. 'SOHUYLEB, m. Attesting Officer Gomissioner of Patmtfl

